The first of the economic forces the rising price ofconventional sources particularly those employing fossil fuels.continues automatically, in part because the resource islimited.

The second reducing the cost of electricity from solar cellsystem is the subject of world wide research anddevelopment efforts today.

To increase the economic attractiveness of the solar cell option:

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increase cell efficiencies

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reduce cost of producing cells modules.

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devise new cell or system designs for lower total costper unit power out put.

Semiconductors:

Semiconductors are crystals that in their pure stateare resistive, but when the proper impurities are addedthis process is called doping in trace amounts oftenmeasured in parts per billion, display much lowerresistance along with other interesting and usefulproperties. Depending on the selection of impuritiesadded, semiconductor materials of two electricallydifferent types:

n-type and p-type.

p-n junction:

The basic structure formed by the intimate contact ofp-type and n-type semiconductors

n-type semiconductor:

A semiconductor type in which the density of holes inthe valence band is exceeded by the density of electronsin the conduction band.

P-type semiconductor :

A semiconductor type in which the density of electronsin the conduction band is exceeded by the density ofholes in the valence band.

The solar cell system work

The most important physical phenomena employed in allsolar cells are very similar to the classical p-n junction.When light is by the junction the energy of the absorbedphotons is transferred to the electron and hole both free tomove. These particles diffuse through the semiconductorand ultimately encounter an energy barrier that permitscharged particles of one sign to pass but reflects those ofthe other sign.

The charge carriers in the junction region create apotential gradient, get accelerated under the electric fieldand circulate as the current through an external circuit.

The current from the cell may pass directly through the loador it may be changed first by the power, conditioningequipment from those provided by the cell, other subsystemsthat may also be used include energy storage devices suchas batteries and concentrating lenses or mirror that focus thesunlight onto a smaller to and hence less costlysemiconductor cell.

Performance of solar cells

An important feature of solar cells is that the voltage ofthe cell does not depend on its size, and remains fairlyconstant with changing light intensity. However, the currentin a device is almost directly proportional to light intensityand size.

Figure below shows example I / V curves for a single cell as a functionof light input

A solar cell's power output can be characterized by twonumbers a maximum Open Circuit Voltage Voc measuredat zero output current and a short circuit current Isc

Where:

Voc = k T/ q ln [(IL

/Io)+1]

And

I = Io

[ exp.(qv/kT)-1]-

I

L

IL

= q AG (Le+ W + Lh)

And the power can be computed via this equation:

P = I * V

As you might then expect, a combination of less thanmaximum current and voltage can be found thatmaximize the power produced. This condition is called"maximum power point”

of silicon has14electrons,arranged in three different shells. The outer shell, however,is only half full, having only four electrons. A silicon atom willalways look for ways to fill up its last shell. To do this, it willshare electrons with four of its neighbor silicon atoms.except that in this case, each atom has four hands joined tofour neighbors. That's what forms the crystalline structure,and that structure turns out to be important to this type of PVcell.

A solar cell has silicon with impurities other atoms mixedin with the silicon atoms, changing the way things worka bit. We usually think of impurities as somethingundesirable, but in our case, our cell would not workwithout them. These impurities are actually put there onpurpose. Consider silicon with an atom of phosphoroushere and there, maybe one for every million siliconatoms. It still bonds with its silicon neighbor atoms, butin a sense, the phosphorous has one electron thatdoesn't have anyone to hold hands with. It doesn't formpart of a bond, but there is a positive proton in thephosphorous nucleus holding it in place.

This diagram shows a typical crystalline silicon solar cell

In solar cells applications this characteristic is usuallydrawn inverted about the voltage axis. The cell generatesno power in short-circuit or open-circuit. The cell deliversmaximum powerPmax

when operating at a point on thecharacteristic where the productIV

is maximum. This isshown graphically below where the position of themaximum power point represents the largest area of therectangle shown.